This invention relates to an electric motor which can constantly measure the transmission torque which is produced during the transmission of torque from the stator to the load.
In FIG. 1, a conventional-type of the above-mentioned electric motor is shown.
In the drawing, numeral 1 indicates an annular stator and such stator is fixedly secured to a motor frame 2. Numeral 3 indicates a rotor and such rotor 3 is fixedly secured to a first hollow shaft 8 which has both ends thereof rotatably supported in brackets 4, 5 by means of bearings 6 and 7. Numeral 9 is an output shaft which is rotatably mounted on the output-end of the first hollow shaft 8 by means of bearings 10, while numeral 11 indicates a second shaft which is concentrically disposed in the first hollow shaft 8 and has one end thereof fixedly secured to the hollow shaft 8 by means of, for example, a shrink fit and other end connected to the output shaft 9. Numeral 12 indicates a circular detecting disc which has at least one detecting element 13a such as a permanent magnet mounted on the periphery thereof. Numeral 14 indicates a circular detecting disc which is provided with at least one detecting element 13b in the same manner as that of the circular detecting ring 12. Numerals 15a and 15b indicate detectors which are designed such that they generate an output signal when the two detecting elements 13a and 13b come in alignment with respective detectors 15a and 15b. Numeral 16 indicates a base provided on the motor frame 2 and such base has a desired number of mounting apertures 17. Numeral 18 indicates a support stay which extends axially from the bracket 5 so as to support the two detectors 15a and 15b in place, while numeral 19 indicates a protective cover. L indicates a set-up size which is a distance between the center axis of the mounting aperture 17 and the extremity of the output shaft 9.
With the above construction, when the motor is driven, the rotating torque of the rotor 3 is transmitted to the output shaft 9 by way of the first shaft 8 and the second shaft 11. Since the first shaft 8 has sufficient bending rigidity against deflection caused by the unbalance of the rotor and the magnetic force and thereby also has sufficient torsional rigidity against torsion, when the load is applied to the output shaft 9, the second shaft 11 is twisted by a suitable twisting angle. Such twisting provides a time lag which is proportional to the above twisting angle between a detecting time where the detecting element 13a provided on the detecting disc 12 of the first shaft 8 comes in alignment with the detector 15a and a detecting time where the detecting element 13b provided on the detecting disc 14 of the output shaft 9 comes in alignment with the detector 15b. The transmission torque is finally measured by utilizing this time lag.
However, such conventional electric motor has the following defects.
(1) Since the second shaft 11 is an elongated bar having a small diameter to permit the twisting thereof, the shaft is easily influenced by the bending moment applied thereto. Therefore, when a considerable degree of bending moment is applied, the output shaft on which the detecting element is mounted is deflected considerably provided that there exists a clearance at the bearing portion disposed between the second shaft and the output shaft.
Accordingly, there is a good chance that an error will occur in the alignment of the detecting element with the detector, resulting in inaccurate torque measurement.
(2) Since the transmission torque is measured by the twisting angle of the second shaft 11, it becomes necessary to arrange at least the detecting element 13b and the detector 15b of the output shaft at the load (output) side. Therefore, such elements are subjected to the heat of the load side which adversely affects these detecting element. Furthermore, since the load side requires a considerable space, the set-up size L between the center-axis of the mounting aperture 17 of the base 16 to the extremity of the output shaft 9, becomes incompatible with the set-up sizes of standard electric motors. Therefore, in replacing such standard electric motors, a conventional motor provided with torque measuring means has to change or renew the mounting position relative to an apparatus which is to be operated by the motor.
(3) In the twisting route from the detecting element 13a of the first shaft 8 to the detecting element 13b of the output shaft 9 by way of the first shaft 8, the second shaft 11 and the output shaft 9, there exist so many mechanical junctions or connections which may cause errors in measuring the transmission torque. Therefore, high precision is required in manufacturing the related parts.
Accordingly, it is an object of the present invention to provide an electric motor which can resolve the aforementioned defects of conventional electric motors and which can allow the output shaft to have enough mechanical strength while assuring an accurate measuring of the transmission torque.
In summary, the present invention relates to an electric motor comprising a stator, a rotor disposed concentrically within the stator, a drive shaft rotatably supporting the rotor with a circular gap between the stator and the rotor and an output shaft connected to the drive shaft and sharing the same rotating axis with the drive shaft, wherein the improvement is characterized in that the electric motor further includes a resilient connecting member which connects the drive shaft and the output shaft and means which detects the torsion of the resilient connecting member produced during the transmission of the torque from the rotor to the output shaft and measures the transmission torque from the detected torsion.